1. Field of the Invention
The present invention relates to an anisotropic magnetic material-dispersed resin carrier, which is mixed with a toner to constitute a developer for developing electrostatic images, and relates to an electrophotographic developer, and a developing device.
2. Description of the Related Art
Image formation by, for example, electrophotography, electrostatic recording or electrostatic printing is generally performed through a process including: forming a latent electrostatic image on a latent electrostatic image bearing member (hereinafter may be referred to as a “photoconductor” or “electrophotographic photoconductor”); developing the latent electrostatic image with a developer to form a visible image (toner image); transferring the visible image onto a recording medium such as paper; and fixing the visible image on the recording medium to form a fixed image.
The developer includes a one-component developer in which a magnetic or non-magnetic toner is used singly, and a two-component developer containing a toner and a carrier.
In general, carriers contained in such a two-component developer are roughly classified into conductive carriers such as iron powder and so-called insulating carriers. The insulating carriers are made to have a high resistance by coating particles such as iron powder, nickel or ferrite with an insulating resin or dispersing fine magnetic particles in an insulating resin.
Carriers having low resistance leak the potential of latent images to fail to obtain good developed images, and are required to have a resistance equal to or higher than a certain resistance. Therefore, conductive carrier cores are preferably coated with an insulating material in use. As carrier core materials, ferrites are preferably used which have a relatively high resistance.
In general, in case of developers having high magnetic force, magnetic brushes formed of the developer become hard in a developing region where the toner contained in the developer develops latent images, thereby causing brush marks, roughness, etc. which make it difficult to obtain high-quality developed images.
In view of this, ferrites are preferably used for lowering the magnetic force of the formed carrier to obtain high-quality images.
Hitherto, there has been proposed adjusting the saturation magnetization of a carrier to a value of 50 emu/g or lower in order to form high-quality images (see Japanese Patent Application Laid-Open (JP-A) No. 59-104663). This proposal can form good developed images having no image failures called brush marks. The lower the saturation magnetization of a carrier is, the better the reproducibility of thin lines is. As being distanced from the magnetic pole, the carrier problematically adheres in higher degrees to a latent electrostatic image bearing member (e.g., a photoconductor drum) (hereinafter such phenomenon is referred to as “carrier adhesion”).
Also, there has been proposed using as a carrier so-called hard ferrite having a coercive force of 300 gauss or more (see Japanese Patent Application Publication (JP-B) No. 04-3868).
However, a developing device is inevitably enlarged to use as a carrier hard ferrite having a high coercive force. It is preferable to employ a developer carrier using a fixed magnetic core in order to realize a compact high-quality color copier. In the compact copiers, the hard ferrite carrier having a high coercive force is poor in transferability due to its self-aggregation property.
In order to solve these problems, there has been proposed a magnetic material-dispersed resin carrier where fine magnetic particles are dispersed in a binder resin, as a carrier preventing carrier adhesion and forming high-quality images (see Japanese Patent (JP-B) No. 3005119). The magnetic material-dispersed resin carrier has low specific gravity to form soft magnetic brushes. It is improved in highlight reproducibility especially in the developing method using an alternating electric field.
However, this magnetic material-dispersed resin carrier cannot sufficiently achieve both desired formation of high-quality developed images and desired prevention of carrier adhesion. Although using this carrier is advantageous in terms of the cost for apparatuses, it still has a problem in achieving both desired solid image uniformity and desired carrier adhesion when used in a developing unit using a direct electric field weaker than an alternating electric field.